1. Field of the Invention
The present invention relates to a method and apparatus for calibrating a non-impact printer for printing information with small pixels (dots).
2. Description of the Prior Art
Various non-impact recording or printer heads for use as dot printers are known. Examples of such heads include an electrostatic printer head, an LED (light emitting diode) array printer head, ink-jet printer head, thermal printer head, etc. While the invention will find utility with regard to non-impact printers in general, discussion will be made herein with regard to LED printer heads with which the invention is particularly suited.
In U.S. patent application Ser. No. 07/290,002, filed in the names of Pham et al, an LED printer is described in which a series of LED's arranged in a straight line are selectively activatable for brief periods to form dot-like images on a recording surface. In the printer described in this application, grey scale recording is achieved by pulsewidth modulation of the exposure times. To accomplish this, a digital comparator is associated with each LED. At one input to the comparator, there is provided the data in the form of a plurality of digital data bits representing an exposure on-time for that LED for that PEL period. At a second input to the comparator, an input from an up/down counter is provided that is rapidly changing in accordance with signals emitted from a high speed clock. As the counter output value decreases towards zero at some point in time a match is sensed by the comparator between the two inputs of the comparator (in accordance with its operating criterion). The LED then turns on and remains on for a very brief duration until a subsequent sensing of a match by the comparator occurs during a count-up phase of the counter. The above application notes that the data may be adjusted to provide also for exposure balancing of the LED's. This is desirable where non-uniformity in illumination from LED to LED on the printer head may be expected. The application notes that a programmable clock may be employed to adjust the time period for each clock pulse.
A problem addressed in the above application is that of efficiently covering as much exposure space as might be desirable for grey level recording. Assuming one employs six data bits to define each pixel, 63 levels of grey may be recorded (not including white, i.e., no activation.) However, these 63 levels must be recordable within a time period comprised of at most 126 uniform clock periods plus a minimum established time, T.sub.MIN. Where the clock periods are of relatively short duration, the 63 levels of grey may not provide the desirable range of grey levels. On the other hand, where the clock periods are of relatively longer duration, adequate extremes of grey may be recorded but fine differences in grey level recording particularly desired at relatively lower grey values will be lost.
The printer described in the above application overcomes this problem by employing a non-linear clock. Thus, clock pulses are generated at nonuniform times to provide an expanded exposure time range without increasing the amount of data bits used to determine exposure time for each LED.
It is an object of the invention to provide an apparatus and method for automatically determining the non-linear clock periods for use in such a printer apparatus.